A fungus is attacking the plane trees along the Canal du Midi, a waterway in southeast France so picturesque that UNESCO called it a 'work of art' and declared it a World Heritage Site.

Reporting from Toulouse, France — For nearly 200 years, the plane trees have stood sentry over the Canal du Midi. Some rise ramrod-straight and proud over Europe's oldest man-made waterway. Others lean like creaky old men, forming an impenetrable canopy over the dappled, barely moving water below.

Their shade protects travelers from the relentless Midi sun. Their roots hold up the canal's banks. Their hardy leaves sink to the bottom and stop the water from seeping into the soil. Perhaps just as important, they transform a utilitarian artery into a thing of natural beauty.

The trees make stretches of the 155-mile canal in southeastern France so picturesque that in declaring it a World Heritage Site, UNESCO said it was not only "one of the most remarkable feats of civil engineering in modern times," but a "work of art."

Now, however, the canal's 42,000 plane trees are under threat. A fungus — allegedly introduced by U.S. servicemen carrying munitions boxes made of diseased sycamore during World War II — is attacking the trees.

For five years, the blight has spread along the waterway, defying attempts to cure or control it. Specialists say it is almost inevitable that all the planes will have to be chopped down, burned and replaced over the next 20 years.

Sitting in his neat, airy office near the banks of the canal in Toulouse, where it starts, Jacques Noisette acknowledges the prospect is heartbreaking.

"When I was told we would have to cut down the trees, I was sick to the heart and sick to the stomach," says Noisette, a spokesman and historian for Voies Navigables de France, the government body that runs the canal, for 21 years. "This isn't like working on a road or a motorway. This is our canal; we feel very strongly about it and hold it in very great affection."

It was just over a mile upstream in Toulouse in January 1667 that the first of an estimated 250 million cubic feet of earth and rubble were dug out to create the celebrated waterway.

Today, flanked by the busy roads, hotels and housing developments of Toulouse, the sunken towpath with its sheltering planes and heavy whiff of jasmine harks back to a peaceful, slower age. Water tumbling through the locks drowns out the relentless drone of traffic.

From Toulouse the canal meanders in a southeasterly direction through the historic towns of Castelnaudary, Carcassonne, Beziers and Agde, to the Thau lagoon and the Mediterranean port of Sete.

The idea of creating a waterway as a shortcut between the Atlantic and the Mediterranean, linking up originally with the River Garonne and, later, the Canal de Garonne, was a Holy Grail for successive French rulers since Roman times.

The route overland was slow, uncomfortable and riddled with bandits. The alternative was a 1,860-mile, one-month voyage by sea around the hostile Spanish coast, dodging storms and pirates.

In 1516, King Francois I brought Leonardo da Vinci to France to survey a possible route, but even the great Renaissance master was unable to solve the fundamental problem of finding a permanent source of water.

Finally, in 1654, wealthy salt tax collector Pierre-Paul Riquet drew up a plan to channel mountain streams into a reservoir to feed the dreamed-of canal.

He was given royal assent in 1666, and work began on the extraordinary canal that nobody believed possible. Riquet was said to be stubborn, solitary and tenacious, and so he proved, creating bridges, aqueducts and tunnels of remarkable scale and engineering complexity to go over, under or through obstacles.

The finished waterway, boasting 91 locks, 126 bridges, six dams and 55 viaducts along with tunnels and aqueducts, is astonishing even by modern standards.

The French military hero Marechal Vauban, architect of many of France's most noteworthy 17th century forts and fortifications, described the Canal du Midi as "without doubt the most beautiful and most noble construction of its kind ever undertaken." He added: "I would have preferred to have created it than all that I have done and all that I will do."

In its citation, UNESCO said the canal had "provided the model for the flowering of technology that led directly to the Industrial Revolution and the modern technological age."

As the planes flourished, they became an integral part of the canal's beauty. The waterway was finally closed to commercial transport in the 1980s and became a tourist attraction, drawing an estimated 2 million visitors a year.

In a world obsessed with speed and pressed for time, traveling down the canal at a sedate 5 mph is an exercise in enforced tranquillity. The technical splendor of bridges, locks and tunnels, and the historical wonder of medieval fortifications and Roman towns are interspersed with stretches of unspoiled pastoral beauty.